Method of making electrical resistors



United States Patent METHOD OF MAKING ELECTRICAL RESISTORS William F. Ebliug, Jr., Springfield, and Cornelius Y. D.

Huang, Upper Darby, Pa., and Tony M. Immediata,

Wilmington, Del., assignors to International Resistance Company, Philadelphia, Pa.

Filed Nov. 28, 1962, Ser. No. 240,556 6 Claims. (Cl. 29155.69)

The present invention relates to a termination for electrical resistors, and more particularly to an electro-mechanical bond between a metal terminal and an electrical resistor body.

One well known type of eletcrical resistor comprises a compressed body of a mixture of particles of an electrically conducting material, such as carbon, metals or mixtures thereof, and an electrical insulating material, such as a ceramic or plastic. The resistor bodies are usually in the form of a solid cylinder, and have metal terminal wires electrically and mechanically secured to the ends of the body. Heretofore, the terminal wires were secured to the resistance body by either inserting the wires into the ends of the body with the body with the body molded around the wires, or by a metal cap secured to the end of the wire and fitting over the end of the body. One problem in securing the terminals to the resistance body by these methods is to insure a strong mechanical connection between the terminal wires and the resistance body, and obtain a good electrical connection between the terminal wires and the electrical conducting material in the body. Another problem with these types of terminal wire connections is that the terminals extend across a portion of the ends of the resistance body so as to electrically short out a portion of the resistance body. Thus, the electrical active length of the resistance body is shorter than the physical length of the body. With the trend toward miniaturization of electrical components, it is desirable to electrically utilize the entire length of the resistance body.

It is an object of the present invention to provide a novel termination for an electrical resistor.

It is another object of the present invention to provide a novel termination for an electrical resistor of the type in which the resistance body is a compressed body of a mixture of particles of an electrically conductive material and an electrical insulating material.

It is a further object of the present invention to provide a termination for an electrical resistor having a good electro-mechanical bond between the terminal Wire and resistance body.

It is a still further object of the present invention to provide a novel termination for an electrical resistor which permits the electrical utilization of the entire physical length of the resistance body.

Other objects will appear hereinafter.

For the purpose of illustrating the invention there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

FIGURE 1 is an elevational view of the resistor of the present invention.

FIGURE 2 is a sectional view taken along line 22 of FIGURE 1.

In general, the termination of the present invention comprises a layer of an electrically conductive metal coated across each end of the resistor body to which terminal wires are to be connected. Prior to coating the ends of the resistor body, which comprises a compressed mixture of particles of an electrically conductive material and an electrical insulating material, portions of the insulating material of the body at the ends of the body 3,237,286 Patented Mar. 1, 1966 are removed to form pockets or cavities, and to expose the electrically conductive particles at the ends of the body. When the layer of the electrically conductive metal is applied to the ends of the resistor body, the metal fills the pockets or cavities to form a strong mechanical bond between the metal layer and the resistor body. In addition, since the particles of the conductive material in the body are exposed, the metal layer makes good electrical contact with the conductive particles. The metal layer is applied to the resistor body by a coating process which is carried out at a relatively low temperature so as not to destroy either the conductive material or the insulating material of the resistance body and to prevent the formation of a nonconducting film on the conductive material. The terminal wires are then soldered to the metal layer on the resistance body.

Referring to the drawing, the resistor of the present invention is generally designated as 10. Resistor 10 comprises a resistance body 12 which is a compressed mixture of particles of an electrically conductive material 14, such as carbon, metals, or a mixture thereof, and an elec trical insulating material 16 such as a ceramic or plastic. Although the resistance body 12 is preferably in the from of a solid cylinder, it may be of any other desired shape. As shown in FIGURE 2, the ends of the resistance body 12 are each provided with pockets or cavities 18 in the insulating material 16 around the conductive particles 14 so as to expose the surfaces of the conductive particles 14 at the ends of the resistance body 12.

Each end of the resistance body 12 is coated with a layer 20 of an electrically conductive metal. The metal layers 20 fill the pockets or cavities 18 in the ends of the resistance body 12, and coat the exposed surfaces of the conductive particles 14 at the ends of the resistance body 12 so as to make electrical contact therewith. Since the metal layers 20 fill the pockets or cavities 18 in the ends of the resistance body 12, there is provided a strong mechanical bond between the metal layers 20 and the resistance body 12.

Each of the metal layers 20 is coated with a layer 22 of an electrically conductive solder. A separate terminal wire 24 of an electrically conductive metal is secured to each end of the resistance body 12. As shown, each terminal wire 24 has an enlarged, flat head 26 on its end which abuts against and is secured to a solder layer 22. Thus, each of the terminal wires 24 is mechanically secured and electrically connected to an end of the resistance body 12 through a solder layer 22 and a metal layer 20.

The resistor 10 of the present invention is made in the following manner:

The resistor body 12 may be formed by any method Well known in the art. For example, the electrically conductive particles 14 and the electrical insulating materials 16 are thoroughly mixed together, and the mixture is then either molded or extruded into the desired shape. The shaped resistance body 12 is then either fired or cured according to whether the electrical insulating material 16 is a ceramic or plastic material. The ends of the resistor body 12 are then treated to form the pockets or cavities 18. This is achieved by contacting the ends of the resistor body 12 with a material which will etch away the electrical insulating material 16, but will not attack the electrically conductive particle 14. If the electrical insulating material 16 is a ceramic, the etching material may be fluoboric acid, hydrofluoric acid or ammonium bifluoride. If the electrical insulating material 16 is a plastic, the etching material may be any suitable solvent for the particular plastic being used, such as acetone, dimethylformamide or toluene. After the ends of the resistor body 12 are etched to form the pockets or cavities 18,

the resistor body is thoroughly Washed to remove all traces of the etchant.

The ends of the resistor body 12 are then coated with the layer 20 of the electrically conductive metal. The layer 20 is applied to the ends of the resistor body 12 by a coating process which is carried out at a relatively low temperature so as not to damage either the electrically conductive particles 14 or the electrical insulating material 16 of the resistor body. One method of coating the ends of the resistor body 12 with the metal layers 20 is to expose the ends of the resistor body 12 to a vapor of a material containing the metal which will decompose at a relatively low temperature, such as a metal carbonyl. The metal carbonyl vapor is heated to its decomposition temperature so as to decompose the vapors and deposit the metal on the ends of the resistor body 12. Another method of coating the ends of the resistor body 12 with the metal layers 20 is by exposing the ends of the resistor body to the vapors of a metal which is evaporated in a vacuum. When the metal vapors contact the ends of the resistor body, the metal will condense on the ends of the resistor body to form the metal layers 20. A third method of coating the ends of the resistor body 12 with the metal layers 20 is by one of the well known processes of electroless plating. Such electroless plating processes are described in U.S. Patents No. 2,996,408 to R. M. Lukes, issued August 15, 1961, No. 3,011,920 to C. R. Shipley, Jr., issued December 5, 1961 and No. 3,033,703 to F. N. Schneble, Jr., et al. issued May 8, 1962.

The terminal wires 24 are then secured to the metal layers 20 by means of the solder layers 22. This may be achieved by first coating each of the metal layers 20 with a layer 22 of solder, placing the head 26 of a terminal wire 24 against the solder layer 22, and heating the solder layer 22 to bond the terminal wire to the metal layer 20. This may also be achieved by holding the head 26 of the terminal wire 24 against an end of the resistor body 12 with a preformed disc of solder being between the head 26 of the terminal wire and the metal layer 20. The solder disc is then heated, such as by induction heating, so as to bond the terminal Wire to the metal layer 20.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

We claim:

1. A method of making an electrical resistor comprising the steps of forming a compressed body of a mixture of particles of an electrically conductive material and an insulating ceramic material, firing said body, then contacting a surface of said body with an etching material which attacks the ceramic material but not the conductive particles so as to remove some of the ceramic material to form pockets in said surface of the body and expose the surfaces of the conductive particles at said surface of the body, then coating said surface of the body with a layer of an electrically conductive metal so as to fill said pockets with said metal, and then securing a terminal wire to said metal layer.

2. A method in accordance with claim 1 including the steps of coating the layer of the electrically conductive metal with solder, and bonding the terminal to said solder.

3. A method in accordance with claim 1 in which the surface of the body of the resistor is coated with the layer of the electrically conductive metal by exposing said surface to the vapors of a carbonyl of said metal which will decompose at a temperature below the decomposition temperature of the body, and heating said vapors to its decomposition temperature so as to deposit the metal on said surface.

4. A method in accordance with claim 1 in which the surface of the body of the resistor is coated with the layer of the electrically conductive metal by evaporating the metal in a vacuum and exposing said surface to the vapors of the metal to condense the metal on said surface.

5. A method in accordance with claim 1 in which the surface of the body of the resistor is coated with the layer of the electrically conductive metal by electroless plating.

6. A method of making an electrical resistor comprising the steps of forming a compressed solid cylindrical body having substantially flat end surfaces of a mixture of particles of an electrically conductive material and an insulating ceramic material, firing said body, then contacting said end surfaces of the body with an etching material which attacks the ceramic material but not the conductive particles so as to remove some of the ceramic material to form pockets in said end surfaces and expose r the surfaces of the conductive particles at said end sur faces, then coating each of said end surfaces With a layer of an electrically conductive metal so as to fill said pocket with said metal, coating each of said metal layers with solder, and bonding a separate terminal wire to the solder on each of said metal layers with the terminal wires extending longitudinally from the end surfaces of said body.

References Cited by the Examiner UNITED STATES PATENTS 1,978,323 10/1934 Power 29155.71 2,037,951 4/1936 Thomson 29-155.71 2,219,365 10/1940 Janssen 29-155.71 2,547,406 4/1951 Morin 338-322 2,681,944 6/ 1954 Comstock 33 8-322 2,796,505 6/ 1957 Bocciarelli 338--329 X 2,994,945 8/ 1961 Cahill 29155.7l

JOHN F. CAMPBELL, Primary Examiner.

RICHARD M. WOOD, WHITMORE A. WILTZ,

Examiners. 

1. A METHOD OF MAKING AN ELECTRICAL RESISTOR COMPRISING THE STEPS OF FORMING A COMPRESSED BODY OF A MIXTURE OF PARTICLES OF AN ELECTRICALLY CONDUCTIVE MATERIAL AND AN INSULATING CERAMIC MATERIAL, FIRING SAID BODY, THEN CONTACTING A SURFACE OF SAID BODY WITH AN ETCHING MATERIAL WHICH ATTACTS THE CERAMIC MATERIAL BUT NOT THE CONDUCTIVE PARTICLES SO AS TO REMOVE SOME OF THE CERAMIC MATERIAL TO FORM POCKETS IN SAID SURFACE OF THE BODY AND EXPOSE THE SURFACES OF THE CONDUCTIVE PARTICLES AT SAID SURFACE OF THE BODY, THAN COATING SAID SURFACE OF THE BODY WITH A LAYER OF AN ELECTRICALLY CONDUCTIVE METAL SO AS TO FILL SAID POCKETS WITH SAID METAL, AND THEN SECURING A TERMINAL WIRE TO SAID METAL LAYER. 